1. Field of the Invention
The present invention relates to a control device for a lock-up clutch of a vehicle torque converter and for a clutch in a starting device or the like that does not use a hydraulic power transmission such as a torque converter. More particularly, the present invention relates to a control device for controlling clutch slip, i.e. controlling a clutch in a state of slipping or “slip state”.
2. Description of the Related Art
In general, an automatic transmission mounted in a vehicle or the like includes a hydraulic power transmission such as a torque converter for providing power transmission while absorbing a rotational speed difference between output rotation of an engine and input rotation of an automatic speed change mechanism, in order to prevent engine stoppage (“killing the engine”) upon stopping or starting of the vehicle. Such a hydraulic power transmission necessarily produces a rotation transmission loss after starting of the vehicle in motion. For improved fuel economy and the like, providing a lock-up clutch capable of locking the output shaft of the engine with the input shaft of the automatic speed change mechanism (that is, locking the pump impeller to the turbine runner of a hydraulic power transmission) has become conventional.
Engagement control of the lock-up clutch, not only includes control of lock-up but also so-called slip control, that is, maintaining a slip state in order to implement both improved fuel economy and riding comfort (reduced engagement shock). The slip control state exists during the ON/OFF transition period of the lock-up clutch. In this slip control, it is preferable, from the viewpoint of reduction of engine vibration transmission and prevention of variation in transmitted torque, to control the amount of slip by controlling a supplied oil pressure so that the rotational speed difference between the output shaft of the engine and the input shaft of the automatic speed change mechanism equals a desired target value. However, accelerator operation by a driver is not necessarily always constant and, when the engine torque varies, it is difficult to control the amount of slip at a target value simply by feedback control only.
In view of the above problem, slip control has been proposed in which, not only is a feedback value calculated based on a target rotational speed difference and an actual rotational speed difference, but also a feed-forward value based on an engine torque is calculated, and slip control is performed using the sum of the feed-forward value and the feedback value (see Japanese Patent No. 2985102). Slip control using an H X controller has also been proposed in which a property change due to a transient operating state (disturbance) is approximated with a high-order function and design of constants of two weighting functions for response and stability is optimized (see Japanese Patent No. 3098667).
In recent years, slip control in a higher engine speed, higher engine torque region has been demanded for control of the lock-up clutch, in addition to that provided during the ON/OFF transition period of the lock-up described above, i.e. during shifting of an automatic speed change mechanism or the like in order to reduce shift shock.
However, at such a higher engine speed, in a higher engine torque region, sensitivity of actual rotational speed difference to an oil pressure value based on variation in engine speed and variation in torque, is relatively small as compared to that at a low engine speed, low engine torque region. Therefore, even if slip control is conducted as taught in Japanese Patent No. 2985102 or Japanese Patent No. 3098667, a delay (lag time) of about several seconds will occur before the amount of slip converges with a target value dependent on variation in engine output, making such control impractical. In actual applications, therefore, slip control is prohibited when the engine speed reaches a predetermined upper limit or when the torque reaches a predetermined upper limit.